Tassel stonewort (Tolypella intricata)
|Size||Stem length: up to 40cm|
Classified as Endangered in the UK, and protected under the Wildlife and Countryside Act 1981 as amended.
Stoneworts used to be classified as members of the plant kingdom, but it is now agreed that they belong, along with other green algae, in the kingdom called Protista. Put simply, the protistas are simple multi-celled or single celled organisms, descended from some of the earliest life-forms that appeared on Earth. Some of the Chlorophytes, specifically the stoneworts, are thought by scientists to have been the early ancestors of all plants. Stoneworts do indeed resemble plants, are frequently mistaken for them, and are often found as fossils. The main body of the plant consists of a series of 'giant cells' up to several centimetres in length, which effectively makes up the stem, together with branches resembling leaves radiating out from nodes that are made up of smaller cells. The stonewort anchors itself, not with roots like a plant, but with rhizoids, colourless, hair-like filaments. Like roots, these can absorb nutrients, but the organism can absorb and breathe through its entire surface. They live in fresh or brackish water, which is low in nutrients and many species require water that is high in calcium. Stoneworts are often encrusted with white lime deposits, giving a crusty texture (hence the name 'stonewort'), and they often have an unpleasant smell, similar to stale garlic.
The Tolypella genus of stoneworts differ from those belonging to the genus Chara, in that they have a different structure. The 'stem' is not covered by lines of small cells to form a 'cortex' as with the Chara stoneworts, but is smooth and consists of a single, highly elongated cell between each whorl of branchlets. In the genus Tolypella the fertile branchlets are small and curved back strongly towards the stem to form dense heads, resembling 'an untidy birds' nest'. Tolypella intricata usually has a moderate amount of lime encrustation, and is a grey or yellowish-green in colour. It differs from the more common clustered stonewort Tolypella glomerata by its sharply-tipped branchlets, and from great tassel stonewort T. prolifera by its smaller size (stem diameter is less than one millimetre).
Tassel stonewort has a scattered range across Europe from Scandinavia to the Mediterranean, and extends into North Africa. Since 1970, it has been recorded in eight localised areas of the UK, all in southern England. In three of these areas there are several colonies within 16 kilometres of each other; ten colonies in one of the Gloucester areas, nine in Oxfordshire and four within half a kilometre in Cambridgeshire. Elsewhere, there are isolated single colonies in Gloucestershire, Suffolk, Norfolk, Somerset and Worcestershire.
This stonewort is found in alkaline water with low nutrient levels, usually in small water bodies that dry out in summer. These include pools, ditches and wheel ruts in tracks. In Ireland, it also grows at the edge of canals. This species appears to benefit from keeping the edges of the ponds or ditches where it has been found well trampled by cattle.
Tassel stonewort seems to prefer small habitats that have been disturbed. It appears quickly but may not last long on a particular site as it is not a competitive species. It behaves as a winter or spring annual, and seems able to cope with cold and frosty conditions, usually producing ripe spores as early as April or May. By early July, the stonewort has usually disappeared. Spores can remain dormant in the mud for many years until suitable open conditions return. It seems to require a period of drying out when water levels drop, in most summers at least.
The end of traditional pond or ditch management practices, combined with the lack of associated disturbance, are the most likely cause of this species' decline. Nutrient enrichment has also led to the loss of some sites.
The tassel stonewort is listed as a priority species in the UK Biodiversity Action Plan (UK BAP), and is included in English Nature's Species Recovery Programme. It is also part of Plantlife's 'Back from the Brink' project. Conservation work currently involves surveying the species' former sites to discover whether it is still present and, where possible, influencing management to conserve the stonewort. The main threat to existing colonies is overshading by scrub and other tall vegetation, and this requires clearance and maintenance work.
The good news is that survey work has resulted in the re-discovery of more sites with populations of tassel stonewort. It is also hoped that, on some sites, it may be possible to re-introduce cattle grazing, which will result in the ditch or pond side trampling that the species seems to like.
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- Brackish: slightly salty water.
- Spores: microscopic particles involved in both dispersal and reproduction. They comprise a single or group of unspecialised cells and do not contain an embryo, as do seeds.
- Whorl: in animals, the spiral or convolutions in the shell of a snail. In plants, a set of leaves, flowers, or branches that spring from a stem at the same point and encircle it.